Traditional land-line based telephone systems connect one telephone set to another through one or more switching centers, operated by one or more telephone companies, over a land-line based telephone network. Traditionally, a telephone connection is based on a circuit switched network. To make a telephone connection, a dedicated circuit or channel is set up for the call between two telephone sets to allow the signal transmission on the dedicated channel during the phone call.
Advanced telephone systems may also use a packet switched network for a telephone connection. A packet switched network is typical in a computer data environment, in which data packets are individually routed over data links that might be shared by many other data communication devices.
Recent developments in the field of Voice over IP (VOIP) allow the delivery of voice information using the Internet Protocol, in which voice information is packaged in a digital form in discrete packets rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN).
Voice over Internet Protocol (VOIP) based telephone calls can be transmitted through a broadband Internet connection instead of a regular (or analog) phone line. Some VoIP services only allow a user to call other people using the same service, but others may allow a user to call anyone who has a telephone number, such as local, long distance, mobile, and international numbers. Typically, a VoIP telephone call is made on a terminal connected to a data communication network, such as a computer that runs a VoIP application on a network connection (e.g., DSL or local area network), a stand alone VoIP phone connected to a communication network, or a traditional phone with an adaptor connected to a network connection.
In a typical VoIP communication, an Analog to Digital Converter (ADC) converts analog voice signal to digital data, which is sent through a data transmission network in data packets in a compressed format using Internet Protocol. At the receiving side, the received data packets are disassembled for the extraction of the digital data. A digital to analog converter (DAC) then converts the digital data back to analog voice signal.
Cellular networks allow a cellular phone to connect to a nearby cellular base station through an air interface for wireless access to a telephone network. Recent developments in wireless telephone systems allow not only voice communications but also data communications. For example, cellular phones can now receive and send short messages through a Short Message Service (SMS). Web pages can now be retrieved through wireless cellular links and displayed on cellular phones. Wireless Application Protocol (WAP) has been developed to overcome the constraints of relatively slow and intermittent nature of wireless links to access information similar or identical to World Wide Web.